In the lithographic fabrication of semiconductor components, such as, e.g., DRAM memory chips, use is made of masks whose structure is transferred onto a target substrate, e.g., a wafer, by means of a light-sensitive resist.
Since the structures to be transferred are becoming smaller and smaller, it is necessary to work with ever shorter exposure wavelengths, such as, e.g., 157 nm or 13.5 nm (extreme ultraviolet or EUV). The requirements made of the corresponding lithographic masks thus change as well. At exposure wavelengths in the EUV range, reflection masks are used instead of transmission masks.
At the ever-shortening exposure wavelengths, diffuse background scattered light (flare) leads to an undesirable reduction of contrast when reflection masks are used. The reduction of contrast leads to a decrease in the size of the process window.
In this case, the scattered light intensity is inversely proportional to the square of the exposure wavelength, i.e., the scattered light increases to a very great extent as exposure wavelengths decrease.
Thus, given the same surface roughness of the lenses or mirrors used as optical elements, the effect for EUV technology at 13.5 nm is more than a hundred times greater than at 157 nm.
The atomic roughness of the optical surfaces represents a theoretical minimum since a minimal flare level of 8% is to be expected in the case of EUV technology.
Since EUV technology uses reflection masks and not transmission masks, it is not possible to use rear-side antireflection layers (ARC) for reducing the influence of flare.
The influence on the process window can be reduced, under certain circumstances, by local adaptation of the critical structure dimensions (critical dimension, local biasing), whereby variations in the flare over the entire image field are not corrected, however. It is also possible to correct the local flare variations by means of different bright field portions of the mask only with a high data-technological outlay. One example of compensation of the changes in the CD that are produced by flare is described in PCT Publication WO 02/27403 and corresponding U.S. Pat. No. 6,815,129.
Furthermore, time-dependent influences such as, e.g., an alteration of the imaging optics during operation cannot be taken into account with this method.
In general, the difficulty exists that transmission mask concepts cannot readily be applied to reflection masks since, particularly, the oblique incidence of light in the case of reflection masks leads to shading effects. All structures on a reflection mask must, therefore, be made as flat as possible.